In a semiconductor manufacturing process, for example, during execution of an etching process, a film thickness of a semiconductor film on a substrate changes with time to decrease. During execution of a thin-film formation process, a film thickness of a semiconductor film changes with time to increase. In such a semiconductor process, for process control such as detection of an end point of the process, etc., in-situ measurement of a temporal change in film thickness of the semiconductor film is necessary.
As such a measurement method for the film thickness of a semiconductor film, there is used a method in which a semiconductor film is irradiated with measurement light with a predetermined wavelength, and interfering light generated by interference of reflected light from the upper surface and reflected light from the lower surface of the semiconductor film is detected. In this method, when the film thickness of the semiconductor film changes, an optical path length difference between the reflected light from the upper surface and the reflected light from the lower surface changes. Therefore, by utilizing a temporal change in detected intensity (interfering light intensity) of the interfering light corresponding to the change in optical path length difference, film thicknesses of the semiconductor film at each time point can be measured.
For example, the film thickness measurement apparatus described in Patent Document 1 disperses transmitted light or reflected light from a to-be-measured object by spectroscopic means to detect interference fringes, and calculates a film thickness of the to-be-measured object from an output for an extremal value when a difference between the maximum value and minimum value of the detected outputs in terms of each wavelength is a predetermined value. Patent Document 2 describes a technique, in a semiconductor thickness non-contact measurement apparatus which irradiates a measuring portion with a light beam from a wavelength variable laser, and detects signal light due to reflected light or transmitted light obtained from the measuring portion, for varying the wavelength of the wavelength variable laser while detecting the intensity of signal light, obtaining a phase change amount from an obtained waveform of changes in light intensity, and obtaining, based on this phase change amount, a semiconductor thickness from a relational expression between the absolute value of a semiconductor thickness and the phase change amount of the signal light intensity.